The ATI Radeon 5770 is a step above your 9800GTX+ and only costs $150. However, it's not a huge difference, just a moderate one.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814102898
The Radeon 5830 would be a substantial upgrade- but you need a good SLI/Crossfire-capable power supply for that card (minimum 500W with a pair of 6-pin PCI-E power connectors, not just one). The Radeon 5830 goes for about $200 before rebates.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150497
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121367
If you've got a 450W power supply with a single PCI-E power connector, the best card you can install is the Radeon 5770. The 5830 or Nvidia's GTX 460 would require a power supply upgrade.
It's a myth that ATI cards work better with AMD or that Nvidia works better with Intel CPUs. There is nothing at the processor level that affects compatibility with either manufacturer's video card.
The biggest result of AMD purchasing ATI is that we've seen fewer high-end AMD motherboards which support SLI, as their emphasis has shifted to Crossfire support for ATI cards. But for single-card configurations, either manufacturer's cards work fine with either company's CPUs.
http://www.tomshardware.com/reviews/geforce-gtx-460-gf104-fermi,2684-11.html
http://www.techspot.com/review/249-ati-radeon-hd-5830/page5.html